Obsidian is a type of glass created when rhyolitic magma rapidly cools. In the Oki Islands, it is mainly found in the coastal areas of Dōgo Island in rock formations from 5.5 million years ago. Although quartz is also present in rhyolite and has nearly the same chemical makeup, obsidian is formed when the glass components collect and solidify without crystallizing. Another condition necessary for obsidian to form is a low water content. If the water content is high, perlite or pitchstone is formed instead, different from obsidian in both appearance and name.
While obsidian is typically black, impurities in its chemical makeup can turn it red, green, or translucent. As a natural glass, it breaks easily with edges sharp enough to cut. For this reason, it was once used to make stone tools.